Coating a porous surface evenly with a liquid can be problematic, due to the tendency of pores to absorb liquids. Coating can be especially troublesome with a liquid that needs to harden or cure. If the porous surface absorbs the coating material before or while it hardens or cures, the surface of the hardened coating material can have uneven thickness, and sometimes uneven coverage of the porous surface. The absorptive properties of pores are due to at least in part to capillary action of the pores, and sometimes also to surface tension of the liquid.
Artificial membranes can be used to perform separations on both a small and large scale, which makes them very useful in many settings. For example, membranes can be used to purify water, to cleanse blood during dialysis, and to separate gases. Some common driving forces used in membrane separations are pressure gradients and concentration gradients. Membranes can be made from polymeric structures, for example, and can have a variety of surface chemistries, structures, and production methods. Membranes can be made by hardening or curing a composition.
In some settings, it is necessary or desirable to form a membrane on a surface of a porous material. For example, industrial membranes are often prepared on porous or microporous supports. The support must be porous to allow permeation of the desired components through the membrane. However, due to the tendency of pores to absorb the composition before it hardens or cures, the resulting texture and thickness of the membrane produced after curing can be inconsistent, making the membrane inadequate for its intended purpose.
Masking the pores with an impermeable interlayer poses a problem with mask removal. Likewise, sealing the pores with wax or other higher viscosity materials makes it difficult to recover the porosity of the substrate after coating, particularly with small pores.